For an accurate calculation of a disparity image or depth image from a stereo camera, the orientation of the two cameras relative to each other must be known. Errors in pitch angle between the two cameras causes one camera to look more downward compared to the other. A roll angle means a rotation of an image plane around the corresponding optical axis. Both these angles must be known in order perform accurate stereo matching between left and right images. For an automotive stereo camera, these angles will not be constant over the vehicle life time due to thermal changes and the long life time of automotive systems. Therefore, an online solution for estimating the pitch and roll angle errors is needed.
A conventional method for estimating pitch and roll angles which is known as 5-point method or feature point method involves finding, for example, five distinctive points, called feature points, in a left and right image pair, and estimating the pitch and roll angle errors from these feature points. This method, however, requires the finding of feature points in the images, and will not work well for example in monotonous, low-light or rainy environments where distinctive feature points cannot be found.